He's the guy who wasted a decade writing an ASCII-art based "template layout module"[1], which he was never able to persuade a single browser vendor to implement. It eventually got demoted from a working draft to a "note", which is the W3C's polite term for standards fanfiction.
His other major achievement is putting his own photo on every one of the CSS working group's web pages. I'm not joking: https://www.w3.org/Style/CSS/#endmatter .
And let me tell you tale of that photo/byline. It originally appeared on the working group's crappy 1990s site, which persisted until the late 2000s (because that's how fast things move at the W3C), and you could kinda forgive it as one of the dumb things people put on dumb 1990s websites.
Anyway, eventually it became too embarrassing that the working group responsible for CSS had such a shitty site, and some other members of the group took it upon themselves to produce a shiny new design with useful content, but sans Bos' photo. This done, they submitted what they'd come up to the W3C. Eventually, it showed up on the W3C's servers but, surprise, surprise, it had been rather mangled, and Bos' photo was back, clumsily shoved into a footer where it hadn't originally been.
It's a small, petty thing, but all the more hilarious for it. God knows how that guy still has a job at the W3C. Maybe he's best friends with Tim Berners-Lee, or something?